Directional coupler



May 14, 1957 w. c. BACKSTRAND 2,792,550

' DIRECTIONAL COUPLER Filed May 3. 1946 MAIN LINE OUTPUT FIG. 2'A

D m WR TT S EK m B C N O T S m W ATTORNEY 2,792,559 DRECTIONAL CGUPLERWinston C. Backstrand, Ithaca, N. Application May 3, 1946, Serial No.666,877 Claims. (Cl. 333-) This invention relates to electromagneticwave coupling apparatus and more specifically to that type of apparatusknown as a directional coupler.

A directional coupler is a device which, when inserted in a transmissionline on which there exist waves travelling in both directions, deliversto a pair of terminals located in an auxiliary transmission line avoltage which is largely-a function of the amplitude of the wave goingin one preferred direction, and relatively independent of the wave goingin the opposite direction.

The part of the directional coupler which forms a section of thetransmission line in which the coupler is inserted will be referred toas the main line. it possesses a set of terminals referred to as themain line input terminals, at which the wave travelling in the preferreddirection enters. The other set of main line terminals will be calledthe main line output terminals.

The auxiliary line contains, in addition to the auxiliary line outputterminals, a resistive termination. The latter serves to absorb thewrong way energy, and to provide a good match looking into the auxiliaryline output terminals. The main and auxiliary lines are connectedtogether by a coupling mechanism which gives the coupler its directionalproperties.

The coupling of the directional coupler specifies that fraction of thepower proceeding in the preferred direction which is delivered to theauxiliary line output terminals. More precisely, the coupling, which maybe expressed either as a voltage or power ratio, is defined as the ratioof the voltage (or power) delivered to a matched detector at theauxiliary line output terminals to the voltage (or power) delivered tothe couplers main line input terminals, provided that the main lineoutput terminals are terminated by a matched load.

The directivity of a directional coupler is defined as the ratio of thevoltage (or power) delivered to a matched detector at the auxiliary lineoutput terminals under the two conditions: (1) power is fed in the main'line input terminals, with a matched load attached to the main outputterminals; and (2) the same amount of power is delivered to the mainline output terminals, with a matched load attached to the main lineinput terminals.

The present invention concerns a slotted block coaxial line directionalcoupler which consists of a solid metallic lock adapted to receive andform a juncture for the main and auxiliary coaxial line sections. Twoquarter wavelength sections of coaxial line having parallel axes areformed in the solid block, and coupling between these lines is providedby two slots in the solid block which are errectively waveguidesoperating below cut off frequency where negligible energy transmissionoccurs. A small amount of the mainline energy is thus coupled into theauxiliary coaxial line, and by spacing the slots apart a quarterwavelength of energy in the coaxial line, a coriventional two-couplingmultiple path directional coupler is formed.

It is commonly desired to increase the coupling between the main andauxiliary coaxial lines. This may be done 17%,556 Patented May 14, 1957by increasing the size of the coupling slot at the iuncture between thewaveguide sections and the coaxial lines, but such increase is limitedby interference requirements and the size of the coaxial linesthemselves. The present invention sets forth simple means for increasingthe coupling between the main and auxiliary coaxial lines of a slottedblock directional coupler.

A principal object, therefore, of the present invention is to provide aslotted blocked coaxial line directional coupler having relatively tightcoupling between main and auxiliary lines.

An additional object is to provide a directional coupier which is simpleto construct with high dimensional tolerances.

Another object gether two coaxial transmission tral axes.

These and further objects will be apparent upon consideration of thisspecification and its accompanying drawings of which:

Figures 1 and 2 are partially cut-away perspective views of apparatus ofthe present invention; and Figure 2A shows an optional form ofdielectric member for the apparatus of Figure 2.

Referring now to Figure 1, a solid metallic block 19 is shown withcylindrical recesses 11 and 12 disposed therein and having parallelcentral axes. With cylindrical conductors 19 and 20 centrally retainedin recesses 11 and 12, two parallel coaxial lines are formed.

Symmetrically cut into faces 14 and 15 of block 10 are recesses 16 (onlyone shown). With cover plates 17 secured in place, the recesses 16 formtwo waveguide sections which couple together cylindrical recesses 11 and12. By properly spacing recesses 16 a quarter wavelength is to provide ameans of coupling tolines having parallel cenof energy travelling in thecoaxial lines formed in block 10, a conventional multiplepathdirectional coupler is formed.

Substantially identical coaxial lines 21 and 22 having a common centralaxes are secured to the metallic block structure shown and form the maintransmission line, while coaxial lines 23 and 24 are similarly securedto form the auxiliary lines. The main line output and input terminals oflines 21 and 22 respectively and the directional coupler output terminalof line 26 are indicated in Figure 1, and a matched dissipative member(not shown) terminates line 24. The center conductors of the coaxiallines shown are supported by the stub chokes 25.

The directional properties of such a coupler depend upon simpleinterference, constructive in the preferred forwarddirection anddestructive in the backward direction. Consider energy to be travellingin both directions in the main line 21, 22. That energy which progressesin the preferred direction, from right to left, is coupled into theauxiliary line through both recesses 16. The preferred direction energyis combined constructively in auxiliary'line 23 as it travels in theri-ght-to-left direction and proceeds to directional coupler outlet.Preferred direction energy travelling from left to right in auxiliaryline 24 is absorbed in the matched dissipative load termination orcancels itself by destructive interference. The energy travelling in thewrong-way direction is also coupled through recesses 16 into theauxiliary line. That part of the coupled wrong-way energy travelling inthe auxiliary line from left to right is absorbed by the linetermination. That portion of the coupled wrong-way energy travellingfrom right to left is canceled by destructive interference, and thedirectivity of the entire coupler is largely terminated by thecompleteness of elimination of such energy.

The dimensions of the recesses 16 are such that they act as waveguidesbeyond cut-oh" and thus direct only a small sample portion of the mainline energy to the auxiliary line, thus making the apparatus shownhighly adaptable to directional power monitoring problems. Because ofthe extremely high attentuation of a cut-off waveguide, it is oftendesired to increase the coupling between the main and auxiliary lines.As stated above, this may be done by increasing the size of the slotsjoining the recesses 16 and the coaxial lines formed in the block 10,but such techniques have serious limitations.

The present invention sets forth directional coupler apparatus whereinthe coupling between the main and auxiliary lines is increased by properdesign of the recesses 16. Heretofore, these cut-off waveguide sections(16) were made with an electrical slot Width w not in excess of thediameter of the cylindrical cavities 11 and 12. In the apparatus shown,tighter coupling is achieved by increasing the physical dimension w soas to permit the greater flow of energy from main to auxiliary lines.

Another form of the present invention is shown in the partial view ofFigure 2. Metallic block 10, cylindrical recesses 11 and 12, centerconductors 19 and 20 and cover plate 17 are identical to thecorresponding elements of Figure 1. In Figmre 2, however, the waveguidecoupling recesses cut into face 14 of block 10 (only one recess shown)has a physical width w equal to the diameter of cylindrical cavities 11and 12. Disposed in these recesses are members 30 (one shown) made of adielectric material such as, for example, polystyrene. These dielectricmembers 30 cause the electrical width of the waveguide coupling recessesto be elfectively made greater than the actual physical width w, and, asstated above, such increase in width provides the desired increase inelectrical coupling between the main and auxiliary transmission lines.

In Figure 2A, a second possible form of the dielectric member is shownwhich is interchangeable with the members 30 of Figure 2. This member 35fills the entire waveguide coupling recess and, in addition, extendsinto coaxial line cavities 11 and 12. Any interruption of energy flowcaused by these members does not cause appreciable back reflectionbecause of their quarter wavelength spacing. Openings 36 in member 35accommodate coaxial line center conductors 19 and 20.

It will be obvious to those skilled in the art that the coup 'ngstructure described in this specification is adaptable to a greatvariety of electromagnetic wave apparatus in addition to the directionalcoupler. The scope of this invention shall not be limited to thespecific details set forth above.

What is claimed is:

1. A directional coupler for the transfer of a portion of the microwaveenergy propagated in a main transmission line to an auxiliarytransmission line comprising, a metallic block having formed therein twoquarter wave length sections of transmission lines positioned to haveparallel axes, said block being recessed at each end of saidtransmission line sections to provide coupling apertures therebetween,means for coupling said main transmission line and said auxiliarytransmission line to said quarter wave length sections of line, andsolid dielectric tween said main and auxiliary transmission lines.

2. A directional coupler for the transfer of a portion of the microwaveenergy propagated in a main transmission line to an auxiliarytransmission line comprising, a metallic block having formed therein twoquarter wave length sections of transmission lines positioned to haveparallel axes, said block being recessed at each end of said sections toprovide coupling apertures therebetween, the width of said recessesbeing equal to the cross-section dimension of said transmission linesections, means for coupling said main transmission line and saidauxiliary transmission line to said transmission line sections, andsolid dielectric inserts positioned in said recesses for increasing theelectn'cal width of said recesses to increase the coupling between saidtransmission lines.

3. A directional coupler for the transfer of a portion of the microwaveenergy propagated in a main transmission line to an auxiliarytransmission line comprising, a metallic block having formed therein twoquarter wave length sections of transmission lines positioned to haveparallel axes, said block being recessed at each end of said sections toprovide coupling apertures therebetween, the width of said recessesbeing equal to the cross-section dimension of said transmission linesections, the depth of said recesses being such as to form a wave guidesection operating below cut-off frequency, means for coupling said maintransmission line and said auxiliary trans mission line to saidtransmission line sections, and solid dielectric inserts positioned insaid recesses for increasing the electrical width of the wave guide soformed to increase the coupling between said auxiliary transmission lineand said main transmission line.

4. A directional coupler for the transfer of a portion of the microwaveenergy propagated in a main transmission line to an auxiliarytransmission line comprising, a metal block having two parallel facesseparated by a quarter wavelength of the propagated energy, said blockhaving a pair of cylindrical transmission line sections with parallelaxes bored between said faces, a cover plate for each of said faces,each of said faces having a recess cut therein forming with said coverplate a wave guide section to couple said transmission line sections,means or coupling said main transmission line and said auxiliarytransmission line to said transmission line sections and a soliddielectric insert having a dielectric constant greater than that of airpositioned in each of said recesses for increasing the electrical widththereof to increase the coetficient coupling between said transmissionline sections.

5. A directional coupler for the transfer of a portion of the microwaveenergy propagated in a main transmission line to an auxiliarytransmission line comprising, a metal block having two parallel facesseparated by a quarter wave length of the propagated energy, said blockhaving a pair of cylindrical transmission line sections with parallelaxes bored between said faces, a cover plate for each of said faces,each of said faces having a recess cut therein forming with said coverplate a wave guide ing the electrical width thereof to increase thecoefficient of coupling between said transmission line sections.

References Cited in the file of this patent UNITED STATES PATENTS2,197,123 King Apr. 16, 1940 2,423,390 Korman July 1, 1947 2,531,777Marshall Nov. 28, 1950 2,562,281 Mumford July 31, 1951

